This application pertains to the art of bootstrap loading systems for digital computers, and more particularly to bootstrap loaders which facilitate replacement or modification of their non-volatile constituent.
The invention as particularly applicable to computer systems employed in relatively hostile or industrial environments, especially those in which functional operation is critical. The invention will be described with particular reference thereto, although it will be appreciated that the invention has broader application such as in any computer system in which a safe, programmable, reboot sequence is advantageous.
Present computer systems must be brought from a "cold" or off state to a state from which usable functionality may be achieved. That is to say, a computer will have no ability to accomplish such functions as interacting with its environment or running of software applications until sufficient operating system or kernel software has been loaded.
Commencement of the software loading process is accomplished by a series of levels of increasing complexity. The way which a computer utilizes such a sequence to get started is analogized to picking itself up by its bootstraps. Hence the term "boot" has been applied to the process. The first level is provided in non-volatile, random access memory which commences at a default address. Upon power up or reboot, a computer is hard-wired to commence operation of a software routine at this address. These fixed instructions typically have just enough intelligence to seek out the next tier of the program loading sequence, often stored on rotating media such as a fixed disk. Other systems employ only random access memory ("RAM") but still require a bootstrap loading sequence of increasing complexity.
Since at least the first level of bootstrap software must be permanently or semi-permanently stored, earlier systems provided this code within a segment of read only memory ("ROM"). Later systems recognized that it is often advantageous or even necessary to update this formerly permanent segment of the bootstrap loading system. Ultraviolet erasable programmable read-only memory ("UV-EPROM") provided a first level of rewritability. Such UV-EPROMs are fabricated with an ultraviolet permeable window. Exposure to UV light of a sufficient intensity and duration functions to erase the chip, allowing for reprogramming of its contents by a dedicated "burner." Another option is permanent substitution of the earlier ROM chip to a chip employing the updated or modified code.
More recent developments in non-volatile memory included electrically erasable programmable read-only memory ("EEPROM"). EEPROMs facilitate semi-permanent or non-volatile memory which may be reprogrammed on an address-by-address basis. Such memories retain their data content when power is removed. A disadvantage associated with EEPROM is a requirement that a substantially increased voltage must be applied to facilitate programming. These increased voltage levels preclude software supervised rewrites from a routine disposed in the same EEPROM for which a write or programming is to be made.
Earlier bootstrap-loading circuits employing EEPROM have addressed the varying voltage level programmability concern in several ways. In a first, the address space of a CPU was divided into a portion on the EEPROM and another portion in a RAM or ROM region. This segmentation facilitated reprogramming under supervision or control of a non-EEPROM constituent program. A disadvantage associated with an ability to reprogram any address space was addressed by use of first and second EEPROM unit, one or both of which could be reprogrammed under the direction of code disposed in the other. Another disadvantage is that a finite number of rewrites are possible with standard EEPROMs.
More recent developments in the area of EEPROM include "Flash EEPROMs." Flash EEPROMs allow for virtually unlimited rewrites with no deleterious effect. However, like earlier EEPROMs, programmability concerns attributed to enhanced voltage levels required therefor are still present.
Computer systems presently find themselves in many industrial environments. Such environments are relatively hostile to electronic components. Such systems are often provided in critical applications or industrial process management or control. Since such systems are often stand-alone systems, sufficient code to reboot them must be provided independently in each. Modification or enhancement of bootstrap loading code would advantageously be accomplished with minimized down time of the associated computer, such as might be realized if entire ROM chips would have to be physically removed. In addition, on-line bootstrap load reprogram may result in corrupted code if a faulty boot code is entered or if a reprogramming sequence is interrupted due to power spikes, failures, error or the like. These problems, which are more common in industrial environments, are exacerbated by the critical nature of such systems and the time consuming process associated with programming EEPROMs.
The present invention contemplates a new and improved on-line bootstrap loader reprogramming system which overcomes all of the above-referred problems, and others, and provides a means by which an entire bootstrap loader ROM base constituent may be replaced with minimal machine down time and minimal risk.